Documentation: cfq-iosched: update documentation help for cfq tunables
Add the documentation text for latency, target_latency & group_idle
tunnable parameters in the block/cfq-iosched.txt.
Also fix few typo(spelling) mistakes.
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Language somewhat modified by Jens.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt
index a5eb7d1..9887f04 100644
--- a/Documentation/block/cfq-iosched.txt
+++ b/Documentation/block/cfq-iosched.txt
@@ -5,7 +5,7 @@
I/O bandwidth for all the processes which requests an I/O operation.
CFQ maintains the per process queue for the processes which request I/O
-operation(syncronous requests). In case of asynchronous requests, all the
+operation(synchronous requests). In case of asynchronous requests, all the
requests from all the processes are batched together according to their
process's I/O priority.
@@ -66,6 +66,47 @@
value of this is 124ms. In case to favor synchronous requests over asynchronous
one, this value should be decreased relative to fifo_expire_async.
+group_idle
+-----------
+This parameter forces idling at the CFQ group level instead of CFQ
+queue level. This was introduced after after a bottleneck was observed
+in higher end storage due to idle on sequential queue and allow dispatch
+from a single queue. The idea with this parameter is that it can be run with
+slice_idle=0 and group_idle=8, so that idling does not happen on individual
+queues in the group but happens overall on the group and thus still keeps the
+IO controller working.
+Not idling on individual queues in the group will dispatch requests from
+multiple queues in the group at the same time and achieve higher throughput
+on higher end storage.
+
+Default value for this parameter is 8ms.
+
+latency
+-------
+This parameter is used to enable/disable the latency mode of the CFQ
+scheduler. If latency mode (called low_latency) is enabled, CFQ tries
+to recompute the slice time for each process based on the target_latency set
+for the system. This favors fairness over throughput. Disabling low
+latency (setting it to 0) ignores target latency, allowing each process in the
+system to get a full time slice.
+
+By default low latency mode is enabled.
+
+target_latency
+--------------
+This parameter is used to calculate the time slice for a process if cfq's
+latency mode is enabled. It will ensure that sync requests have an estimated
+latency. But if sequential workload is higher(e.g. sequential read),
+then to meet the latency constraints, throughput may decrease because of less
+time for each process to issue I/O request before the cfq queue is switched.
+
+Though this can be overcome by disabling the latency_mode, it may increase
+the read latency for some applications. This parameter allows for changing
+target_latency through the sysfs interface which can provide the balanced
+throughput and read latency.
+
+Default value for target_latency is 300ms.
+
slice_async
-----------
This parameter is same as of slice_sync but for asynchronous queue. The
@@ -98,8 +139,8 @@
request.
In case of storage with several disk, this setting can limit the parallel
-processing of request. Therefore, increasing the value can imporve the
-performace although this can cause the latency of some I/O to increase due
+processing of request. Therefore, increasing the value can improve the
+performance although this can cause the latency of some I/O to increase due
to more number of requests.
CFQ Group scheduling